Induced miR-99a expression represses Mtor cooperatively with miR-150 to promote regulatory T-cell differentiation

Peripheral induction of regulatory T (Treg) cells provides essential protection from inappropriate immune responses. CD4 + T cells that lack endogenous miRNAs are impaired to differentiate into Treg cells, but the relevant miRNAs are unknown. We performed an overexpression screen with T‐cell‐expressed miRNAs in naive mouse CD4 + T cells undergoing Treg differentiation. Among 130 candidates, the screen identified 29 miRNAs with a negative and 10 miRNAs with a positive effect. Testing reciprocal Th17 differentiation revealed specific functions for miR‐100, miR‐99a and miR‐10b, since all of these promoted the Treg and inhibited the Th17 program without impacting on viability, proliferation and activation. miR‐99a cooperated with miR‐150 to repress the expression of the Th17‐promoting factor mTOR. The comparably low expression of miR‐99a was strongly increased by the Treg cell inducer “retinoic acid”, and the abundantly expressed miR‐150 could only repress Mtor in the presence of miR‐99a. Our data suggest that induction of Treg cell differentiation is regulated by a miRNA network, which involves cooperation of constitutively expressed as well as inducible miRNAs. Synopsis miRNAs identified in an overexpression screen reveal the importance of coordinated suppression of mTOR expression for promoting Treg induction. Many microRNAs have a positive while others a negative effect on naive T‐cell differentiation into Tregs. miR‐99a and miR‐150 promote Treg differentiation. The highly expressed miR‐150 contributes at physiological levels while the lower expressed miR‐99a is inducible. The neighboring target sites of miR‐150 and miR‐99a in the 3′ UTR of the Mtor mRNA show the importance of cooperative miRNA‐mediated repression. Graphical Abstract miRNAs identified in an overexpression screen reveal the importance of coordinated suppression of mTOR expression for promoting Treg induction.